This application claims the priority of German application 101 16 661.3, filed Apr. 4, 2001, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a device for indicating the total load in a brake disk made of carbon-fiber-reinforced ceramic material, as well as to methods of providing such a device and indicating such a total load.
Brake disks of this type are known from German Patent Documents DE 44 38 455 C1 and DE 198 34 542 A1. German Patent Document DE 44 38 455 C1 shows a method of producing a friction unit in which a porous carbon body is infiltrated with liquid silicium. Here, the silicium, together with the carbon, converts to silicon carbide (SiC). The carbon body is preferably constructed as a carbon fiber body. If a wheel unit, which here is a brake disk, is assembled of two halves, recesses can be made in the carbon bodies before ceramization. These recesses will then, as a result of assembling the carbon bodies, complement one another to form a joint cavity such as, for example, a cooling duct. German Patent Document DE 198 34 542 A1 also discloses a method of producing bodies which contain reinforcing fibers. In contrast to the method of German Patent Document DE 44 38 455 C1, here, fibers, fiber bundles, or fiber agglomerates are used and aligned such that the reinforcing fibers in the area of the recesses in the brake disk are oriented approximately parallel to lateral edges of the recesses.
As a result of high loads, the known fiber-reinforced ceramic materials experience a progressive total loading in the sense of damage; however, no signs of damage are visible to the outside. In particular, it is not possible to detect, as in known brake disks made of ferrous materials, the total load by way of the abrasion of material. In brake disks made of a fiber-reinforced ceramic material, this abrasion of material amounts to only a few grams and, furthermore, is not an indicator of damage to the brake disk.
It is an object of the invention to provide a device for indicating the total load which can be used for brake disks and other friction elements made of carbon-fiber-reinforced ceramic material.
According to the invention, this object is achieved by way of an indicator element provided in the brake disk so as to start from a friction surface of the brake disk and having an oxidation stability which is reduced with respect to the material of the brake disk. The invention is based on a recognition that, with respect to the material of the above-mentioned type for the brake disk, damage occurs mainly by oxidation of the carbon fibers. As a result of a reduction in the oxidation stability in comparison to the material of the brake disk, the indicator element clearly wears more rapidly than the brake disk material, so that the burning-off of the indicator element can be clearly detected and can be used as a measurement of the total load of the brake disk.
Other advantageous features of the invention are reflected in dependent claims.
It is suggested to produce the indicator element of a carbon material. Such a material may, for example, be graphite, moldings made of carbon fiber, sintered carbon materials, or organic binders, as they are used for brake linings. These materials consist of carbon so that, as a function of the temperature, their oxidation behavior corresponds to the oxidation behavior of the carbon fibers of the brake disk. Organic materials, at first, do not consist of pure carbon. However, during the first heatings as a result of braking operations, these materials are converted to carbon due to coking, and their oxidation behavior will again correspond to that of a carbon fiber.
Furthermore, it is suggested to construct the indicator element in the shape of a pin and to insert it into a bore in the brake disk. As a result of this indicator element construction, production and arrangement of the indicator element in the brake disk are simplified considerably because pins and bores, respectively, can be produced in a simple and cost-effective manner. This arrangement is particularly advantageous with a brake disk which is already provided with bores. In such a brake disk, one of the bores, which exists anyhow, can be used for accommodating the indicator element.
Finally, it is suggested to connect the indicator element with the brake disk by a silicon-carbide-containing layer. This preferably takes place by inserting the indicator element into the brake disk before siliconizing the brake disk. The edge layer of the indicator element is also siliconized during the siliconizing process so that a connection layer between the brake disk and the indicator element is formed without any additional process steps. Furthermore, it is possible to insert the indicator element into the formed carbon body while the body is in its preliminary stagexe2x80x94a green compactxe2x80x94rather than later, and to carbonize it together with the green compact. Particularly with an indicator element made of an organic material, this method can be used to also carbonize the organic material and thus establish a defined condition of the indicator element. After siliconizing, care should be taken to free the indicator element from a silicon carbide layer which may be situated in the area of the friction surface of the brake disk in order to create a good working surface for oxidation of the indicator element.
The invention will be described in detail by way of an embodiment illustrated in the figures.